Die casting plunger piston ring



Jan. 31,1967 w. E. THOMPSON DIE CASTING PLUNGER PISTON RING Filed May 5, 1965 United States Patent 3,300,822 DIE CASTING PLUNGER PISTON RING William E. Thompson, Anderson, Ind, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed May 3, 1965, Ser. No. 452,658 3 Claims. (CI. 2270) This invention relates to improvements in pistons for hot chamber die casting and is particularly concerned with providing for lubrication between the piston ring and cylinder wall during the high temperatures and pressure conditions encountered in the die casting process.

As is well known in the art of die casting, molten metal is forced into the die cavity under the pressure exerted by a piston or plunger. In the hot chamber die casting process the piston and pressure chamber are submerged in a reservoir of hot molten metal and the molten charge or shot flows into the chamber through inlet ports. The piston on its pressure stroke closes the ports and forces the entrapped molten metal into the die developing a pressure in the vicinity of 2500 p.s.i. or more. This pressure is maintained on the metal in the die cavity until it has solidified. The piston is then lifted to an intermediate position still blocking the inlet ports of the shot cylinder. This creates a vacuum under the shot plunger so that the molten metal is withdrawn from the sprue. At this point the dies are opened and the piston is raised further opening the inlet ports to the pressure cylinder.

Conventional plunger-ring-pressure cylinder combinations used in die casting utilize a ring which is rectangular in annular cross section. During the charge stroke the fluid pressure forces the ring out against the surface to be sealed. In die casting of zinc, for example, metal injection pressures of 2000 psi. minimum are required and peak pressures sometimes approach 15,000 psi. It can readily be seen that a ring sliding at high velocity over a surface at these pressures would require the best lubrication possible. However, the conventional rectangular ring actually excludes lubrication. These rings have been subject to collapse and leakage after a relatively short operating period due to the intense frictional heat generated on the compression stroke. But even prior to the failure of the piston ring there is a loss of vacuum on the withdrawal stroke due to rapid piston ring wear which allows the excess molten metal to drip into the die area when the dies are opened.

It is an object of this invention to provide a piston for die casting which, in addition to maintaining adequate pressure on the shot during the charge stroke, will control leakage on the withdrawal stroke and will permit selflubrication.

It is another object to provide a piston for die casting with axially spaced compression rings, which under the pressure of the molten metal on the downward stroke will maintain an adequate pressure on the shot, and which will maintain a vacuum on the upward, or withdrawal, stroke whereby the molten metal is Withdrawn from the sprue, said compression rings being chamfered so as to force a film of molten metal between the ring and the cylinder walls thus lubricating the bearing surfaces.

It is a further object to provide a piston for die casting with axially spaced compression rings having one chamfered edge so as to force a film of molten metal between the ring and the cylinder walls, thus lubricating the bearing surfaces and markedly improving the seal characteristics and wear life of the ring.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein preferred embodiments of the present invention are clearly shown.

3,300,822 Patented Jan. 31, 1967 In the drawings,

FIGURE 1 shows a sectional elevation view of a portion of a hot chamber die cast machine including the furnace, reservoir, molten metal, gooseneck, extension nozzle, pressure cylinder, and shot piston in raised position.

FIGURE 2 is an enlarged fragmentary vertical view of the piston and three piston rings.

FIGURE 3 is an enlarged fragmentary vertical view of the subject piston ring and surrounding piston and pressure cylinder wall.

The gooseneck injection apparatus for charging the molten metal into the die cavity of a hot chamber die casting machine is shown in FIGURE 1. A piston 10 operates reciprocally in a vertically disposed pressure cylinder 11. The cylinder 11 is connected to the sprue (not shown) through a gooseneck 12 and extension nozzle 13. The piston 10, cylinder 11, and gooseneck 12 are immersed in a molten metal bath in a reservoir 14. The metal is heated by means of a furnace 15. When the piston is in the fully raised position (as shown in FIG- URE 1) the molten metal may flow into the pressure cylinder 11 and the gooseneck 12 through an inlet port 16.

When the piston or plunger 10 descends it closes the inlet port 16 and molten metal in the cylinder 11 and gooseneck 12 is forced through the extension nozzle 13 and into the sprue (not shown). After the metal in the die cavity (not shown) has solidified the piston 10 is raised to an intermediate position still blocking the inlet port 16 of the cylinder 11. A vacuum is thus created under shot piston 10 and the molten metal is withdrawn from the sprue. The dies (not shown) can now be opened and the piston 10 raised to its uppermost position (as shown in FIGURE 1).

During this metal-injection cycle the pressure and vacuum seal is maintained by one or more piston rings 17. The rings are split at 18 and are located in axially spaced grooves 19 in the piston 10. These grooves 19 are located in the cylindrical surface of the piston and normally lie in a plane perpendicular to the longitudinal axis of the piston. The axial dimension 20 of the groove is slightly greater than the thickness 21 of the piston ring. In addition, the diameter of the bottom of the groove is less than the inside diameter of the piston ring.

Thus, on the pressure stroke the ring will slide to the top side of the groove as shown in FIGURE 3. This leaves a space 22 below and beside the ring into which the molten metal may flow. The high pressure of the molten metal together with the resiliency of the ring force the cylindrical surface of the ring 23 against the wall of the pressure cylinder 11 to effect the seal.

On the withdrawal stroke the ring slides against the bottom surface of the groove and the seal is maintained largely by the resiliency of the piston ring. The effectiveness of the vacuum seal depends mainly upon the resiliency of the rings, the absence of severe wear due to friction, and the combined effect of two or more rings because of the overlapping of ring joints 13. Wear can be controlled most effectively by providing for adequate lubrication.

Conventional die cast piston rings are rectangular in annular cross section. Therefore, on the pressure stroke it is apparent that the lower or leading edge will actually scrape or remove any molten metal from the cylinder wall 11, leaving insufficient lubrication between such a piston ring and the cylinder wall. Under the pressure and velocity of die casting operation tremendous frictional heat is generated. Such heat causes ring failure after a relatively short period of operation and reduces the seal on the pressure and vacuum strokes.

The advantage of the subject invention is that the piston rings 17, as best seen in FIGURE 3, acting in combination with the plunger and the cylinder 11, are tapered 24. A given ring 17 has its outside edge on the pressure side of the ring chamfered 24. The angle of the chamfer from the longitudinal axis of its piston carrier is in the range of 5 to 20. It has been found that in zinc die casting an angle of is optimum. The function of this chamfer is to channel or wedge a thin film of molten metal between the cylinder and surface 23 of the piston ring 17 and the wall of the pressure cylinder 11 thereby effectively lubricating the bearing surfaces and markedly increasing its seal characteristics.

The ring in the combination of the subject invention can vary in thickness 21, but actually need be no more than approximately 4 inch in thickness. The preferred embodiment as described uses three thin rings because it has been observed that a number of thin rings are more elfective than a single thick one in maintaining the seal on the vacuum stroke. The desired depth of the chamfer 25 is a function of the angle of the chamfer and the thickness of the ring 21. However, the depth normally would not be greater than and in the case of .a ring A1" in thickness 21 an effective chamfer depth 25 is 0.030". The chamfered ring must, of course, have sufiicient cylindrical surface 23 to seal against the pressure cylinder wall 11.

While the invention has been described in terms of the hot chamber die casting apparatus, it is apparent that the chamfered ring could also be utilized in combination with a piston and pressure cylinder in the cold chamber die casting apparatus. Therefore, it is to be understood that the scope of the invention is not to be limited except as defined in the following claims.

I claim:

1. In a die casting machine the combination of a pressure cylinder having an inlet port, a shot plunger reciprocably operable within said cylinder and having a circumferential groove in the cylindrical surface thereof disposed perpendicularly to the longitudinal axis of said plunger, and a resilient expandable split piston ring in said groove, said ring having an annular chamfer on the outside diameter of the pressure side whereby molten metal is wedged between the outer cylindrical surface of said ring and the cylinder wall during the pressure stroke of said plunger, the angle of said chamfer being on the range of about 5 to said groove being of greater axial thickness than said ring and the inner diameter of said groove being smaller than the inner diameter of said ring whereby the pressurized molten metal flows into said groove between the base of said groove and the inner side of said ring to exert a sealing pressure on said ring against said cylinder wall during said pressure stroke of said plunger.

2. In a hot chamber die casting machine the combination of a vertically disposed pressure cylinder having an inlet port, a shot plunger reciprocably operable within said cylinder and having a circumferential groove in the cylinder surface thereof disposed perpendicularly to the longitudinal axis of said plunger, and a resilient expandable split piston ring in said groove, said ring having an annular chamfer on the outside diameter of the pressure side whereby molten metal is wedged between the outer cylindrical surface of said ring and the cylinder wall during the pressure stroke of said plunger, the angle of said chamfer being in the range of about 5 to 20, said groove being of greater axial thickness than said ring and the inner diameter of said groove being smaller than the inner diameter of said ring where by the pressurized molten metal flows into said groove between the base of said groove and the inner side of said ring to exert a sealing pressure on said ring against said cylinder wall during said pressure stroke of said plunger.

3. In a :hot chamber die casting machine the combination of a vertically disposed pressure cylinder having an inlet port, a shot plunger reciprocably operable within said cylinder and having a circumferential groove in the cylindrical surface thereof disposed perpendicularly to the lon gitudinal axis of said plunger, and a resilient expandable split piston ring in said groove, said ring having an annular chamfer on the outside diameter of the pressure side whereby molten metal is wedged between the outer cylindrical surface of said ring and the cylinder wall during the pressure stroke of said plunger, the angle of said chamfer being about 15, said groove being of greater axial thickness than said ring and the inner diameter of said groove being smaller than the inner diameter of said ring whereby the pressurized molten metal flows into said groove between the base of said groove and the inner side of said ring to exert a sealing pressure on said ring against said cylinder wall during said pressure stroke of said plunger.

References Cited by the Examiner UNITED STATES PATENTS 1,764,815 6/1930 Williams 277-143 2,913,269 11/1959 Bremer et a1 277236 FOREIGN PATENTS 145,775 3/1952 Australia. 52,799 2/1937 Denmark.

1,300 5/1853 Great Britain.

J. SPENCER OVERHOLSER, Primary Examiner. E. MAR, Assistant Examiner. 

1. IN A DIE CASTING MACHINE THE COMBINATION OF A PRESSURE CYLINDER HAVING AN INLET PORT, A SHOT PLUNGER RECIPROCABLY OPERABLE WITHIN SAID CYLINDER AND HAVING A CIRCUMFERENTIAL GROOVE IN THE CYLINDRICAL SURFACE THEREOF DISPOSED PERPENDICULARLY TO THE LONGITUDINAL AXIS OF SAID PLUNGER, AND A RESILIENT EXPANDABLE SPLIT PISTON RING IN SAID GROOVE, SAID RING HAVING AN ANNULAR CHAMFER ON THE OUTSIDE DIAMETER OF THE PRESSURE SIDE WHEREBY MOLTEN METAL IS WEDGED BETWEEN THE OUTER CYLINDRICAL SURFACE OF SAID RING AND THE CYLINDER WALL DURING THE PRESSURE STROKE OF SAID PLUNGER, THE ANGLE OF SAID CHAMFER BEING ON THE RANGE OF ABOUT 5* TO 20* SAID GROOVE BEING OF GREATER AXIAL THICKNESS THAN SAID RING AND THE INNER DIAMETER OF SAID RING GROOVE BEING SMALLER THAN THE INNER DIAMETER OF SAID RING WHEREBY THE PRESSURIZED MOLTEN METAL FLOWS INTO SAID GROOVE BETWEEN THE BASE OF SAID GROOVE AND THE INNER SIDE OF SAID RING TO EXERT A SEALING PRESSURE ON SAID RING AGAINST SAID CYLINDER WALL DURING SAID PRESSURE STROKE OF SAID PLUNGER. 